Light-emissive electroluminescent devices that employ an organic material for light emission are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene) (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes recombine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as tris-(8-hydroxyquinolino)aluminium (“Alq3”). In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.
The performance of such devices can be enhanced by introducing one or more charge transport layers between the light-emissive layer and one or both of the electrodes. Such charge transport layers are typically formed of one or more thin layers of electrically-conducting polymers. Polymers which have been found to be effective in this role include polyaniline, as described in European patent application 91301416.3, and in Y. Yang and A. J. Heeger, Appl. Phys. Lett. 64, 1245-1247 (1994), derivatives of polythiophene, including poly(3-methylthiophene), as described in S. Hayashi, H. Etoh and S. Saito, Jap. Journ. Appl. Phys. 25, 773-775 (1986), and polystyrene-sulphonate-doped polyethylene dioxythiophene (PEDOT:PSS) available from Bayer AG (Germany) and described in UK patent application number 9703172.8.
One approach to producing a full colour display device is to form an array of pixels that can each emit one of the primary colours: red, green and blue, and to control the pixels in the normal way to produce desired colour patterns by combinations of those colours. To optimise the operation of such a device it is important that the red, green and blue colours are emitted with sufficient spectral purity. This poses considerable technical problems. Patterning of the light-emissive films is, although possible, not easily achieved using the traditional techniques of semiconductor photolithography, because the organic materials are themselves susceptible to the photo-patterning or etching steps conventionally used, although this problem is less severe for polymer films than for sublimed films of small molecules. Another approach, which is described in U.S. Pat. No. 5,294,870 and has, for example, also been demonstrated by C. Hosokawa et al. in ‘Proceedings of the 49th annual conference of the Society for Imaging Science and Technology, May 1996, Minneapolis, USA, page 388 and by S. Tasch et al. in Advanced Materials 9, 33-36 (1997), is to fabricate an array of blue LEDs, and to use a colour conversion medium to convert this blue emission into either red or green. The colour conversion medium is required to function in a fluorescent mode, but can provide relatively efficient energy conversion between colours. However, this structure suffers from the extra fabrication costs associated with the extra processing steps, and is intrinsically less efficient than structures the do not rely on secondary emission. Another of the problems in the use of such layers is that, as currently exemplified, they are ‘passive’ layers, which may have to be placed outside the light emitting device (LED) structure, in the path of the emerging radiation. This can, for example, lead to ‘colour cross-talk’ between pixels due to the thickness of the substrate which is typically positioned between the LED and the colour conversion medium.
F. Cacialli et al., Appl. Phys. Lett 69 (25), 16 Dec. 1996, 3794-3795 describe an organic light-emissive device in which a layer of undoped PPV is found to absorb some light emitted from an underlying emissive layer. However, the electrical properties of the resulting device suffer because there is a significant barrier to onward charge transfer from the PPV layer (e.g. the extra undoped PPV layer increases the drive voltage).
A separate issue is that ambient light incident on a display from its surroundings, for example daylight, can impair the performance of the display. Ambient light can cause unwanted photo-degradation or photo-conduction in components of the display. Some frequencies of ambient light can excite components of the display, especially the light-emissive components, to fluoresce and thus emit unwanted light, reducing contrast.
There is therefore a need for an improved means to permit the fabrication of colour displays, preferably producing purer emission colours and/or multi-colour emission and preferably having good device efficiency and ease of manufacture. There is also a need to reduce problems from ambient light.